The present invention relates to a dispersion flattened fiber (DFF) with high negative dispersion and a method for the manufacture thereof; and, more particularly, to a dispersion flattened fiber with high negative dispersion to be utilized for a dispersion compensation in a conventional single mode fiber (SMF) or a non-zero dispersion shifted fiber (NZDSF) by setting up a dispersion thereof to be high negative, i.e., ranging, e.g., from xe2x88x9220 to xe2x88x9260, not zero, at a wavelength band of 1.55 xcexcm.
In the field of optical communications, dispersion is defined as a pulse-spreading phenomenon caused due to the fact that the wave velocity of an optical signal passing through an optical fiber varies depending on the wavelength thereof.
As a conventional optical fiber for transmission, there exist an SMF optimized for a 1.31 wavelength band and an NZDSF with a small dispersion for 1.55 xcexcm wavelength band, and the like.
However, when the conventional SMF or NZDSF is used, a maximum transmission distance is limited as a transmission speed increases. Generally, the relationship between the transmission speed B [Gb/s] and the maximum transmission distance L is shown as follows:                     L        =                  104000                                    B              2                        xc3x97            D                                              Eq        .                  xe2x80x83                ⁢        1            
wherein D represents a dispersion.
When the SMF (whose dispersion is about 17 ps/nm/km at a wavelength of 1.55 xcexcm) is used to transmit an optical signal at a speed of 2.5 Gb/s, a maximum transmission distance is 979 km according to Eq. 1, but when the transmission speed increases to 10 Gb/s, the maximum transmission distance is diminished to just about 60 km. When the NZDSF (a dispersion thereof is about 2 to 7 ps/nm/km) is used to transmit the optical signal with the transmission speed of 10 Gb/s, the maximum transmission distance is limited to about 148 km. In the case of adopting a wavelength division multiplexing (WDM) transmission method that features high speed and big capacity, a dispersion slope as well as the dispersion must be taken into consideration in order to estimate a maximum transmission distance.
Accordingly, in order to increase the maximum transmission distance at a predetermined wavelength band, it is essential to compensate not only the dispersion but also the dispersion slope. As a solution to this assignment, a dispersion compensation fiber (DCF) has been developed. Although the DCF compensates for both the dispersion and the dispersion slope simultaneously, the manufacturing process thereof is too complicated.
Up to now, researches in the DCF have been mainly focused on a method for flattening the dispersion to be nearly zero at a wavelength band of 1.55 xcexcm.
When the SMF is employed to transmit an optical signal at a transmission speed of more than 10 Gbps, both the dispersion and dispersion slope, which limit directly the maximum transmission distance, must be compensated. This can be achieved by employing a DCF that compensates both the dispersion and the dispersion slope at the same time. However, a manufacture of the DCF has not been easy.
A variety of methods for compensating a dispersion of an optical fiber by using a dispersion compensation module, which comprises DCFs, have been developed. Since it is not easy to produce a DCF capable of simultaneously compensating both the dispersion and the dispersion slope, an alternative method using two separate DCFs for exact dispersion compensation has also been developed as follows:
LDCF1xc3x97DDCF1+LDCF2xc3x97DDCF2+LSMFxc3x97DSMF=0xe2x80x83xe2x80x83Eq.2
                                          S            SMF                                D            SMF                          =                                                            L                DCF1                            xc3x97                              S                DCF1                                      +                                          L                DCF2                            xc3x97                              S                DCF2                                                                                        L                DCF1                            xc3x97                              D                DCF1                                      +                                          L                DCF2                            xc3x97                              D                DCF2                                                                        Eq        .                  xe2x80x83                ⁢        3            
wherein L DCF1, L DCF2 and L SMF represent the maximum transmission distance of a first DCF, a second DCF and a SMF, respectively; DDCF1, DDCF2 and D SMF stand for the dispersion of the first DCF, the second DCF and the SMF, respectively; and SDCF1, SDCF2 and SMF represent the dispersion slope of the first DCF, the second DCF and the SMF, respectively.
In the case of using two different DCFS, it is required to combine the dispersions and the dispersion slopes of the two DCFS, so that the exact compensation for the dispersion becomes more difficult.
It is, therefore, the object of the present invention to provide a dispersion flattened fiber having high negative dispersion as well as flat dispersion characteristic at a transmission wavelength band so as to compensate the dispersion with advanced facility and exactness, and also provide a manufacturing method of such dispersion flattened fiber.
In accordance with a preferred embodiment of the present invention, there is provided a dispersion flattened fiber with high negative dispersion comprising:
a central core;
ring-type cores and low refractive regions alternately formed outside the central core;
a cladding formed surrounding the ring-type cores and the low refractive regions; and
a coating formed outside the cladding so as to protect the central core, the ring-typed cores, the low refractive regions and the cladding.